1. Field of the Invention
This invention relates to a method and an apparatus for forming a casting (a molding) which includes an insert. Examples of such a casting are electric coils or inductors, IC's, and thermistors which have inserts sealed in resin.
2. Description of the Prior Art
Some of known electric coils (electric inductors) and IC's have coatings of potting material or sealing plugs for providing waterproofness. It is known to seal electric parts in resin. Sealing electric parts in resin is advantageous in cost and recycling. Generally, sealing electric parts in resin uses an insert molding process which is good in cost and mass-productivity.
Japanese published unexamined patent application 55-91642 discloses an apparatus for forming a casting which includes an insert. In the apparatus of Japanese application 55-91642, dies have a cavity in which an insert is located, and the insert can be supported by hold pins capable of moving relative to the dies. During the formation of the casting, the insert is located at a given place within the cavity while being supported by the hold pins. Under these conditions, the cavity is charged with molten resin. Then, the hold pins are moved back from the cavity before the cavity is further charged with molten resin. The final casting contains the insert entirely sealed in the resin.
During the formation of the casting by the apparatus of Japanese application 55-91642, if the hold pins are moved back from the cavity before charging the cavity with molten resin is completed, the insert tends to be displaced by a motion of the molten resin. The displacement of the insert causes an unevenness in thickness of the final casting.
It is conceivable that the hold pins are moved back from the cavity after charging the cavity with molten resin is completed. In this conceivable case, solidified layers of resin have already been formed around the hold pins immediately before the hold pins are moved back from the cavity. As the hold pins are moved back from the cavity, molten resin enters and occupies the spaces resulting from the backward movement of the hold pins. The molten resin, which enters and occupies the spaces, tends to insufficiently fuse with the solidified layers. The insufficient fusion causes minute holes in the resin of the final casting which allow the introduction of water or others toward the insert. The minute holes correspond to unfused portions of the resin in the final casting.